The present invention relates to an apparatus and process using an improved nozzle tip for atomizing a liquid stream, optionally together with a gas stream, and introducing the liquid together with the optional gas stream into a vessel where it contacts fluidized particles. In a specific embodiment the invention relates to introducing and atomizing a hydrocarbon feed into a riser reactor used in a fluidized catalytic cracking process.
In modern fluidized catalytic cracking the cracking reaction is effected by introducing the hydrocarbon feed at a lower or upstream, end of a riser reactor conversion zone together with hot fluidizable catalyst particulates. The hot catalyst supplies all or a major proportion of the heat to vaporize the feed and to carry out the endothermic cracking reaction.
The vaporized feedstock and catalyst pass up the riser reactor together at high superficial velocities. Because of the high activity of the catalyst, the cracking reaction has generally proceeded to the desired extent at the upper, or downstream, end of the riser reactor. The cracked hydrocarbons are then separated from the catalyst in a disengaging vessel and are sent downstream for further processing. The catalyst is, in turn, stripped with an inert gas such as steam to remove entrained hydrocarbons before being sent to a regenerating zone for removal of the coke which accumulates on the catalyst during the cracking process. The regenerated cracking catalyst is then introduced into the riser reactor.
Due to the extremely short contact times between the hydrocarbon feed and the fluidized catalyst particulates in the riser reactor, it is highly desirable to achieve immediate and intimate mixing of the hydrocarbon feed and catalyst particulates in order to achieve more uniform conversion of the hydrocarbon within the confines of the riser reactor conversion zone.
It is known that improved mixing reduces undesirable gas make, increases gasoline selectivity, improves the effect of catalytic cracking in preference to thermal cracking and reduces carbon formation.
Further, as refiners have perceived the need to blend heavier feeds, e.g., resids, with the typical fluidized catalytic cracking hydrocarbon feed due to economic incentives or supply constraints, the necessity of obtaining intimate and immediate mixing of catalyst particulates and hydrocarbon feed has become even more important. Specifically, heavier fractions of the heavier hydrocarbon feeds are not as readily vaporized in the riser reactor upon contact with the hot catalyst. Such non-vaporized components do not facilitate the desired intimate contact between catalyst particulates and feed. Liquid wetting of the catalyst particulates reduces the surface area available to catalyze the desired hydrocarbon reactions and results in increased coke formation due to adsorption of the heavy fractions present in the feed or by polymerization. Consequently the process duties of the catalyst stripper and regenerator are increased. Liquid droplets and wet catalyst particulates may also deleteriously deposit as coke on the walls of the riser reactor.
Accordingly, within the context of a short residence time fluidized catalytic cracking process, the feed injection system's ability to effect immediate and intimate mixing of catalyst particulates and hydrocarbon feed coupled with the rapid vaporization of the hydrocarbon feed is of paramount concern. The rate of hydrocarbon feed vaporization in the riser reactor is, of course, increased by increasing the degree of atomization of the hydrocarbon feed charged to the riser reactor.
In this connection it is known to use nozzles to atomize the hydrocarbon feed to the riser reactor. The feed nozzle performance determines the degree of atomization of the hydrocarbon feed which subsequently determines how well the feed mixes with the catalyst.
Generally, feed nozzle technology includes the use of an internal mixing zone for mixing liquid hydrocarbon and optionally an atomization medium gas such as steam followed by passing the mixture through a downstream nozzle tip designed to create small liquid hydrocarbon droplets from the hydrocarbon-steam mixture wherein these droplets can be distributed across the riser in a generally horizontal flat spray pattern. This flat spray pattern is achieved by injecting into the riser from a nozzle entering the riser in a side entry configuration. It is desirable that that the flat spray pattern be thin and uniform having a narrow distribution of fine droplets.
Certain nozzle tips comprise an elongated slot orifice to produce a flat fan spray pattern while other nozzle tips comprise two or more, i.e. multi-orifice designs. While the multi-orifice design provides enhanced atomization, these multi-orifice nozzle tips as well as elongated slot orifice tips are prone to erosion damage from increased erosion from the particulates. This erosion occurs because the jet of liquid and optionally gas passing through the nozzle tip creates a low pressure region near the periphery of the orifice opening and causes fluidized particles circulating in a vessel or reaction zone to flow to this periphery region and erode the nozzle tip. This erosion is more pronounced in multi-orifice nozzle tips. Premature or frequent replacement of nozzle tips in chemical plant or oil refinery units is a costly disadvantage to the operator.
Accordingly there is a need for a nozzle tip that provides greater atomization but mitigates the erosion characteristics associated nozzle tips especially with multi-orifice nozzle tips.